The present invention relates to a device and method for detecting polarization mode dispersion of an optical data signal.
Long optical waveguide transmission links are used in optical transmission technology. Due to Production imperfections, these optical waveguide links are not completely isotropic, but rather weakly birefringent. Therefore, a long transmission link results in frequency-dependent polarization transformationxe2x80x94known as polarization mode dispersion or polarization dispersion, (PMD). Because of the change in the polarization of the optical signal as a function of the optical frequency and the different frequency-dependent delays, this PMD effect leads to the widening of transmitted pulses. This means that at the receiving end, the identifiability of said pulses is reduced and, as a result, the data rate that can be transmitted is limited.
This effect is further aggravated by the fact that the transmission response of the link and hence the PMD, change as a result of temperature variance or mechanical stress. Accordingly, adaptive PMD compensators are inserted in the transmission path. To drive these compensators, PMD distortions must be detected in the optical receiver. The compensator can then be set optimally, for example by using a gradient algorithm.
In Electronic Letters, Feb. 17, 1994, volume 30, No.4, pages 348 to 349, use is made of a bandpass filter for filtering a data signal whose PMD is to be detected. A power detector at the filter output supplies a signal which is higher, the smaller the PMD distortions are.
What is disadvantageous is that when a high degree of first-order PMD is present, this signal does not vary monotonically as a function of the differential group delay (DGD) and, therefore, clear signals cannot be obtained.
In Proceedings OEC 94, 14e-12, pages 258 to 259, Makuhari Trade Fair, Japan 1994, a different method is used, in which the power of the differential signal between decision-circuit output and decision-circuit input is evaluated. However, this signal has less sensitivity to PMD distortions than a suitable bandpass filter. Moreover, incorrect decisions may occur, particularly, in the event of severe PMD distortions, where the DGD exceeds the bit period. As such, the signal obtained is an unsuitable criterion for indicating the presence of the PMD distortions.
The object of the present invention is to specify a reliable detector even for relatively large values of the differential group delay. Furthermore, the present invention specifies a suitable arrangement for compensation of the polarization mode dispersion and the optimum setting of the detector.
The object is achieved by detecting polarization mode means of a device for dispersion including a filter(s) and a measuring device(s) that is connected to the output of the filter(s). The measuring device supplies output signals that correspond to the spectral power densities at various frequencies of the photodetected optical data. The respective deviation between these output signals that correspond to the spectral power densities and each of the output signals in the undistorted condition at different frequencies indicates the signal distortion.
The particular advantage of the invention is in the combination of output voltages of a plurality of filters. These voltages have a monotonic profile in are the principle ranges utilized, and particular high steepness, which is not available with the use of a single bandpass filter or a single low-pass filter. Accordingly, a significantly more accurate compensation is thus possible.
In other words, the use of bandpass filters has the advantage over the use of low-pass filters in that a higher steepness of the filter output voltages as a function of the differential group delay is achieved. As a result, even a more accurate/faster compensation can be carried out.
Instead of a plurality of bandpass filters/low-pass it is also possible to use switchable/controllable bandpass switchable/controllable low-pass filter.
The device for detecting polarization mode dispersion can be further supplemented by a criteria. Devices which evaluate intentionally producedxe2x80x94error rates of a data auxiliary signal obtained from the received optical signal are particularly advantageous in this case. A particularly simple circuit can be realized by means of a controllable sampling threshold in the evaluation of the data signal.